This invention relates to a current detector for detecting a supply current supplied to a load with good accuracy.
Hitherto, a high-accuracy current detector has been demanded for the purposes of finding the charge amount of a secondary battery used with a personal computer, etc., with good accuracy, monitoring a current distribution circuit in an automobile, etc. A method of placing a shunt resistor of a high-accuracy low resistance element in series with wiring through which the current to be detected flows and detecting a voltage drop occurring in the shunt resistor, thereby detecting a current value is known.
For example, in FIG. 10, a current I12 proportional to a load current IL flowing into a shunt resistor Rs is formed using pnp transistors Q11 and Q12 and a resistor R11 and is converted into an output current VOUT using a resistor R12, thereby detecting the load current IL.
Hitherto, a method of using an operational amplifier IC to differentially amplify the potential difference occurring across a shunt resistor of a high-accuracy low resistance element and output the voltage proportional to a load current has been known. In this case, however, the offset voltage of the operational amplifier IC is superposed on the output voltage, thus a problem is involved in the accuracy particularly when the load current is small. In contrast, there is also an operational amplifier IC whose output voltage can be adjusted by connecting a variable resistor to an external terminal, but the operational amplifier ICs are not suitable for mass production. A method of canceling the effect of offset voltage by a circuit shown in FIG. 11 is known.
In FIG. 11, if an operational amplifier OP1 has a sufficiently large amplification factor, the potential difference between an inversion input terminal and a noninversion input terminal of the operational amplifier OP1 can be assumed to be zero. Therefore, EQU I14.multidot.R14=(I13+IL).multidot.RS.apprxeq.IL.multidot.RS
where R14 is the resistance value of a resistor R14, RS is the resistance value of a shunt resistor Rs, I13 is a current flowing into the inversion input terminal of the operational amplifier OP1, and I14 is a current flowing into the noninversion input terminal of the operational amplifier OP1.
Thus, the collector current I14 of a pnp transistor Q13 becomes EQU I14.apprxeq.I1.multidot.RS/R14
and a current proportional to load current IL can be output.
However, in the circuit previously described with reference to FIG. 10, the range in which the voltage proportional to the load current IL flowing into the shunt resistor Rs of a high-accuracy low resistance element is provided is narrow, thus to monitor a small current area and to monitor a large current area, different circuits need to be used, for example, in such a manner that the resistance value of the resistor R12 is changed; this is a problem. In [DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS] covered later, using FIG. 3, comparison with the circuit forming the invention is made for the relationship between the load current and the output voltage in the circuit in FIG. 10.
Use of the circuit previously described with reference to FIG. 11 using the operational amplifier IC requiring a large number of circuit elements is not preferred considering implementing of the current detection circuit in IC form, the manufacturing costs, etc.